The term “biological matrix” denotes a body fluid or a tissue of a living organism. Biological matrices for the purposes of the invention are optically heterogeneous, i.e. they contain a large number of scattering centers, at which incident light is scattered. In the case of biological tissue, in particular skin tissue, the scattering centers are formed by the cell walls and other solid components contained in the respective tissue. Body fluids, in particular blood, also constitute optically heterogeneous biological matrices since they contain particles at which light is multiply scattered.
The transport of light in a biological matrix is essentially determined by the scattering of light at scattering centers contained in the matrix, and by optical absorption. Physical parameters which describe these two properties quantitatively are called light transport parameters (scattering parameters and absorption parameters). In this context, a scattering parameter is primarily the scattering coefficient, μs, and an absorption parameter is primarily the absorption coefficient, μa. However, within the scope of the invention these parameters need not be determined quantitatively in the customary units. Rather, it is the objective of the invention to determine in a reproducible and selective manner a parameter that describes the optical scattering in the biological sample independent of its optical absorption. Hereafter the scattering coefficient, μs, shall be referred to as an example of a scattering parameter without limiting the general applicability of the invention.
The selective determination of the scattering coefficient is of general interest in medicine, since important diagnostic information can be derived from the interaction of light with skin tissue and other biological matrices. As an example, it is possible in dermatology to characterize skin properties by this means.
Particularly significant is the investigation of the scattering behavior of a biological matrix for the purpose of determining, in a non-invasive manner, the concentration of analytes influencing the scattering of light, in particular glucose. The relationship between the glucose concentration and the scattering of light in biological matrices is described in EP 0659055 B1. As is illustrated therein (and in numerous other publications concerned with the analysis of glucose in the human body), the quality of diabetic therapy crucially depends on a frequent, preferably continuous, determination of the time course of the blood sugar level in the body of diabetics. Thereby severe secondary damage due to diabetes mellitus, such as loss of eyesight or severe circulatory disorders possibly leading to a need for amputation of limbs, can be prevented. The desirable continuous monitoring of the blood sugar level is not feasible with conventional invasive methods (in which a droplet of blood is removed from the patient's body and analyzed with one of the analytical systems that are currently available at good quality and favorable costs). Consequently, there have been numerous attempts to determine the concentration of glucose by a non-invasive approach. A more detailed overview is provided in the European patent referred to above.
In the method described in EP 0659055 B1, a plurality of “detection measurements” is performed to determine a glucose value, wherein light is irradiated as primary light into the biological matrix at a defined light irradiation site, the light propagates in the biological matrix along a light path, and an intensity measurement value of the secondary light exiting at a defined detection site is measured. The glucose concentration is determined from the dependence of the intensity measurement value on the measuring distance between the respective light irradiation site and the respective detection site by means of an evaluation algorithm and a calibration.
The surprising finding that a measuring method of this kind can be used to measure the change over time of the glucose concentration in skin tissue or other biological matrix, is in EP 0659055 B1 explained by the fact that the change in the refractive index of the liquid contained in the matrix caused by a changing glucose concentration (although small) leads to a change in light scattering that can be used to determine the glucose concentration by investigating the scattering behavior of light in compliance with the further instructions provided in the European patent. According to a preferred embodiment, the influences of absorption and scattering are separated in the evaluation step by analyzing the intensity distribution of the secondary light as a function of the distance of the detection site from the light irradiation site.
Similarly, it has been discussed in the scientific literature for a substantial period of time to determine μa and μs from the dependence I(ρ) of the intensity, I, of the secondary light from the measuring distance, ρ, (hereinafter referred to as “intensity profile”). The theoretical basis is provided by diffusion theory and numerical statistical methods (Monte Carlo calculations). The theory provides a model for the description of the propagation behavior of light in a scattering matrix, in which a mathematical relationship is established between the intensity profile, I(ρ), and the model parameters used in the model (mainly the light transport parameters, μa and μs, and the intensity of the incident primary light, I0. In principle, it is feasible to determine the light transport parameters by performing a fit, in which the intensity profile calculated theoretically is optimally fitted to the experimental results by varying the parameters of the model. In this context, reference is made to the following publications:    1) T. J. Farrell et al.: “A diffusion theory model of spatially resolved, steady-state diffuse reflectance for the noninvasive determination of tissue optical properties in vivo”, Med. Phys. 19, 879 to 888 (1992)    2) R. C. Haskell et al.: “Boundary conditions for the diffusion equation in radiative transfer”, J. Opt. Soc. Am. A, 11, 2727 to 2741 (1994).
Although the measured values and the theoretical calculations are reported to agree quite well in these references, these methods never attained any practical significance for the determination of the glucose concentration in a biological matrix.
The patent literature describes a number of methods aiming to determine μa and μs in a biological matrix in order to obtain therefrom analytical data for medical purposes, in particular for the determination of the glucose concentration.    3) According to EP 0760091 B1, at least two frequency domain measurements each are performed for at least two different measuring light paths, the measurements including the determination of the phase shift of the secondary light as compared to the primary light and the determination of an intensity measurement value (namely the DC intensity or the AC intensity). An absorption parameter and/or scattering parameter is then derived from these at least four measuring values. Since frequency domain measuring procedures operate with light modulated in the GHz range they require extensive measuring technological resources.)    4) EP 0774658 A2 describes a method, in which the scattering properties of a biological matrix are analyzed by varying the reflection properties at the surface of the matrix. As an example, the contact surface of the measuring head used for the measurement can comprise different partial areas with different reflectivities. By this means, the reflection properties are varied at least two-fold for two measuring distances. The publication illustrates that the resulting at least four measuring values can be used to separate absorption and scattering (either on the basis of diffusion theory or by empirical-numerical means). This method also is relatively resource-consuming. Moreover, it is difficult to achieve the level of reproducibility of the measuring values that is required for the analysis of the glucose concentration.    5) German patent application 10110599.1, dated Mar. 6, 2001, published after the priority date of this patent application, describes a specific evaluation algorithm for the determination of the scattering coefficient from a plurality of detection measurements. The relative change of the intensity profile over time is used to calculate a time derivative value as an intermediary value to simplify the separation of the influences of the model parameters (in particular of the absorption coefficient and the scattering coefficient), and thereby achieve improved accuracy in the determination of the scattering coefficient by simple means.